| constant(adjective, certain, determined, persistent, reliable) |
| iron(noun, adjective, mean, hard, tough) |
| merciless(adjective, appearance, mean, hard, tough) |
| stiff-necked(adjective, mean, strong, hard, tough) |
| tenacious(adjective, determined, resolute, obstinate, uncompromising) |
| pertinacious(adjective, strong, tough, cold, determined) |
| pitiless(adjective, animal, mean, hard, tough) |
| tough(adjective, mean, strong, hard, severe) |
| continuous(adjective, firm, certain, persistent, constant) |
| set in stone(expression, adjective, idiom, mean, hard) |
| steady(adjective, determined, resolute, persistent, constant) |
| unmerciful(adjective, mean, hard, tough, severe) |
| continual(adjective, certain, determined, persistent, constant) |
| mulish(adjective, mean, strong, hard, tough) |
| steely(adjective, strong, hard, tough, determined) |
| uncompliant(adjective, resist, determined, resolute, obstinate) |
| endless(adjective, certain, determined, persistent, constant) |
| hardheaded(adjective, strong, hard, tough, cold) |
| incompliant(adjective, resist, determined, resolute, persistent) |
| pat(adjective, determined, resolute, obstinate, uncompromising) |